So I need to make a matrix of points to spread buttons over an area, I wanted to make a class that has the matrix itself because the buttons wont appear on screen all the time and they will move in the grid etc.

anyway, here`s what the constructor for that class does:

 public Table()
        {

            for (int i = 0; i < 4; i  )
            {
                for (int j = 0; j < 4; j  )
                {

                    _Grid[i, j] = new Point(i * 90, j * 90);


                }
            }
        }

90 is the width of the buttons

then I have a property that allows getting the grid:

 public Point[,] Grid
            {
            get;
            }

then i do the form_load like this:

 private void Form1_Load(object sender, EventArgs e)
        {
            Table tab = new Table();
            Point[,] grid = tab.Grid;

            for (int i = 0; i < 4; i  )
            {
                for (int j = 0; j < 4; j  )
                {

                    Button btn =  new Button();
                    btn.Width = 88;
                    btn.Height = 88;
                    btn.Location = grid[i, j]; // it returns a NullReferernceException here
                    panel1.Controls.Add(btn);

                }
            }   
        }

adding the location gives me a nullreference, but if i copy the code from the constructor and make the grid matrix in form_load get it, it works.(button width and height is 88 so there`s a nice little space between the buttons)

im obviously still learning, and you can see im stuck in the very beginning of this project because i cant even make all the buttons appear where they should.

CodePudding user response：

It's quite a guess, but I suspect you've done something like this in Table

private Point[,] _Grid = new blahblahblah
public Point[,] Grid
        {
        get;
        }

These two things are not referring to the same thing in memory - you've filled _Grid and it's valid object, but the property Grid is referencing some invisible backing store variable created by the compiler; having a name that is similar is not enough to wire them together

• Change Table to be like:

    public Point[,] Grid { get; } = new blahblahblah

• Delete any mention of _Grid in favor of Grid

• Change the constructor so it sets up the array via the property (thus writing values into the invisible backing field created by the compiler):

    Grid[i, j] = new Point(i * 90, j * 90);

And then retrieving tab.Grid will retrieve a populated thing that is not null

Footnote, we name private members like _camelCase, not _PascalCase and if you want to explicitly declare a read only property that gets a class member field, it would look like:

private Point[,] _grid = blahblahblah;
public Point[,] Grid { get { return _grid; }}

Or:

private Point[,] _grid = blahblahblah;
public Point[,] Grid { get => _grid; }

i.e. you have to concretely wire them together by having the prop go and get that field by name, and return it

_{We can get into "properties should not return arrays" another time :D}